In a duplex communication link such as provided in the present invention for a cordless telephone system, one of the main difficulties is in providing for two-way transmission with sufficient selectivity in the receivers to reject transmission from adjacent, co-located transmitters. In one of the prior art duplex communication systems the receiver in the duplex link includes four stages of ordinary IF filtering. The difficulty in this system is that the carrier frequencies must be separated a sufficient amount so that transmitted and received carrier frequencies avoid interference capable of disabling the receivers. Under these constraints of widely separated transmit and receive frequencies, it is difficult to conform to Federal regulations without operating in a licensed frequency band for transmission. Accordingly, in the prior art system, the citizens band is used for one transmission frequency of the duplex link and a lower high frequency (HF) band, separated by several MHz, is used for the other. Due to the radiation restrictions in the lower HF frequency band, the range of the system is unduly limited.
Another difficulty found in the prior art is that maintenance of the connection to the telephone line is dependent upon a continuous tone transmitted from the Mobile Unit which is detected and used to close the telephone line relay when the tone is present. This has been found unsatisfactory because of momentary radio interference or fading of the signal transmission causes an interruption of the telephone connection.